@b0rk realizing that literally nobody "knows it". Everyone just learns the parts that are useful to them.

So learn enough of the shell language to be able to identify what you don't understand, and then pick up stuff as you use it. Cheat sheets and stack overflow are great for finding new tools and getting started, but they're not gonna teach you anything, so read the help docs for the flags you use… and don't worry about the rest. You're not going to be an expert in every tool, that's crazy.